1. Field of the Invention
This invention relates to an electrode for a lithium battery, a method for producing the same, a lithium battery which contains the electrode according to the invention, the use of the electrode in implantable medical devices, and a powder mixture which is used in the electrode production process.
2. Description of the Related Art
In recent years the functionalities of electrically operated medical implants have steadily increased. New therapeutic possibilities have been integrated by new electronic components and even the radiocommunication of implants with external devices have been provided (telemetry function).
However, the demands on the electrical properties of batteries have also increase as a result of the additional functionalities:                The use of batteries with a high energy density is required for miniaturising the implant in order to guarantee adequate life of the device.        For telemetry functions of implants it is also necessary for the batteries to be able to transmit current pulses of adequate intensity and high frequency during the transmission process.        The use of battery cases that are as thin-walled as possible is required to miniaturise the battery and minimise battery weight.        
Furthermore, the battery should swell as little as possible during discharge. Swelling of the battery in a hermetically sealed implant may result in damage to components, e.g. the electronics contained therein. If a swelling battery presses from the inside against the casing of the implant, the implant may be widened undesirably. In the most unfavourable case this may result in stress cracking of the battery or implant casing.
Progressive miniaturisation of the implants requires the use of flat, prismatic batteries with a contour optimised on the basis of physiological considerations. However, prismatic batteries in particular are subject to substantial swelling since a pressure acting from the inside deforms mainly the large area end faces of the casing.
According to the state of the art the swelling of batteries is a well-known phenomenon. A typical cause of battery swelling is a density of the reaction products formed during discharge that is lower than the educts. This gives rise to an increase in volume during discharge, as is the case with lithium/iodine batteries, for example. The formation of gaseous reaction products frequently results in swelling of the battery. The formation of gaseous products in lithium batteries also depends on the moisture content of the components used in the battery. For example, water reacts with the lithium of the anode to form gaseous hydrogen. The swelling of a battery is generally also associated with an increase in impedance. This effect may be caused by gaseous substances which reduce the active surface of the of the electrodes. The swelling may also result in an increase in the distances between the anode and cathode, and hence in an increase in the impedance of the battery. The structure of porous electrodes may also be negatively influenced by swelling if, as a result of the widening of the electrode structure, the electrical contacting of the active materials is impaired by the conductivity additives contained in the electrodes.
The prevention of the formation of gaseous products inside the battery by the use of dried materials and the production of batteries in a dry atmosphere conforms to the state of the art and is also an ideal method for producing implantable batteries. However, these measures alone only give rise to a reduction in swelling.
The addition of V6O13 to the cathode material of lithium/manganese dioxide batteries to remedy the problem is proposed in U.S. Pat. No. 5,308,714, an addition which is able to repress the formation of gaseous reaction products. JP 57003368 discloses the addition of lanthane oxide (La2O3) or yttrium oxide (Y2O3) to the cathode material in order to reduce the swelling of the battery. These additives must be added to a previously heat treated cathode material. Further heat treatment at 100-450° C. is then required. The use of metal oxides such as V6O13, La2O3 or Y2O3 as additives in the active material of the cathode is always associated with a reduction in the energy density of the battery since they do not participate linearly in the electrochemical discharge reaction. Furthermore, the high costs of these compounds and their toxicity restrict their use, particularly in the medical sector.
It is also known that swelling increases in lithium batteries in which copper silver vanadium oxide, copper vanadium oxide or silver vanadium oxide is used as the cathode material if extremely finely divided carbon (soot) (is added to the cathode to improve electrical conductivity. If the proportion of conductivity additives is reduced to a value of 2% graphite and 1% soot, the swelling can be reduced (U.S. Pat. No. 5,569,558). However, the method described has only limited application since a proportion of conductivity additives totaling 3% produces only a slight improvement in the electrical conductivity of the electrode. This substantially limits the loading capacity of the cathodes per unit of area.
EP 0 978 889 describes a method for minimising the swelling by the use of a conductivity additive containing carbon, with a specific area <100 m2/g, which is added to a cathode in addition to finely divided graphite. The use of mixtures of conductivity additives represents an additional expenditure involved in the production of the electrodes because the different components have to be mixed homogeneously with each other and dried. This process is technically all the more expensive the greater the difference between the particle sizes of the materials and the greater the differences between the densities of the materials.
Electrolyte additives which are intended to help reduce the swelling of batteries have been described on several occasions. For instance, KR 1020040085960, for example, discloses that the addition of halogenated aromatic components in a quantity of 0.01 to 10% to the electrolyte will reduce a swelling in a rechargeable (secondary) battery. Electrolyte additives for reducing the swelling may also be used in implantable batteries, but their production and cleaning represent an additional cost.
JP 11265722 describes a method in which the anode material of the battery is used in a particular volume ratio to the cathode material in the battery. This will reduce the swelling of the battery to below a value of 10% related to the total thickness of the battery in that the volume released when the anode is discharged compensates for some of the battery swelling. The matching of the anode and cathode volumes can only represent a technical solution in batteries in which the reduction in volume of the anode is approximately equal, when the battery discharges, to the increase in volume of the cathode or the gas volume formed during discharge. However, in high-energy batteries for implantable devices the volume of the cathode is so high, compared to the anode, that it is impossible for the decrease in volume of the anode to compensate for the swelling of a normal battery cathode.
Design measures on battery casings for reducing the swelling of batteries have also been described. For example, KR 1020050020210 describes a prismatic casing construction from a hardened battery casing which is sealed with a lid from an extremely soft material. This will reduce the welling of the large-area end faces of the prismatic battery because the battery lid preferably bulges outwards. U.S. Pat. No. 6,248,472 also describes the use of a stabilised battery casing to reduce swelling. However, structural reinforcements of the battery casing are unsuitable for implantable batteries because they increase the weight of the battery and result in a reduction in energy density with the same battery volume. In contract a reduction in the material thickness of the casing is required to miniaturise the battery. Moreover, the deflection of the swelling in a spatial direction does not represent an applicable technical solution in hermetically sealed implants because there is insufficient clearance in any direction inside the casing.
According to EP 1 156 541 the use of fibrous cathode materials in batteries will help prevent the swelling of batteries, but the effect is described exclusively in connection with the cathode material carbon monofluoride.